A switching power supply device is mounted in an electronic instrument such as, for example, a portable telephone or digital camera, and is used for raising or lowering an input voltage, and supplying the voltage to a load. With this kind of switching power supply device, a condition relating to a power supply action is set by various methods in accordance with power supply specifications required by the electronic instrument.
As important parameters, such as switching frequency, relating to the switching power supply device affect power supply noise characteristics, part size, part withstand voltage, temperature rise, and the like, a number of setting methods have been conceived. A switching power supply circuit shown in Japan Patent No. JP-A-2007-14082 below raises a direct current voltage supplied from a direct current power source, and supplies the voltage to six white light emitting diodes LED1 to LED6, which are a load, thereby attempting to drive the white light emitting diodes LED1 to LED6. In the switching power supply circuit, an externally attached resistor R1 is provided as an adjusting resistor in an oscillator circuit of a boost chopper regulator in order to adjust the switching frequency.
Normally, when the switching power supply circuit is used in an electronic instrument such as a portable instrument, liquid crystal television, or DVD player, of noise emitted from the switching power supply circuit, there is noise that has an adverse effect on another LSI inside the electronic instrument, and noise that does not. In the invention of the switching power supply circuit shown in Japan Patent No. JP-A-2007-14082, there is a linear relationship between the resistance value of the externally attached resistor R1 and the switching cycle, because of which the designer of the electronic instrument can freely change the switching cycle by replacing one externally attached resistor R1, and adjustment of emitted noise is easy. However, there remains a problem in that, in order to apply the invention to a power supply control IC (integrated circuit) including a boost chopper regulator, an additional dedicated terminal to which the frequency adjusting resistor is connected is necessary.
The number of terminals of a power supply control IC for controlling a switching power supply device is limited owing to requests to suppress cost, package area, and the like. Because of this, it is often the case that no dedicated terminal setting a parameter other than the heretofore described switching frequency is provided in the power supply control IC. In this case, in order to satisfy the power supply specifications of a large variety of electronic instruments, there is no option but to respond by increasing the lineup of the power supply control IC itself.
However, in the event that the lineup of the power supply control IC increases for individual electronic instrument power supply specifications, this also leads to an increase in product cost. Therefore, in order to reduce the cost of the electronic instrument, there is a demand for a product wherein it is possible to select a plurality of necessary kinds of operation condition with one power supply control IC.
As a way of responding to this kind of request, an invention of “A Method and Device that Select a Parameter/Mode Based on Time Measurement”, wherein a setting of an operation condition is carried out during an initialization period of a power supply control IC utilizing a multi-functional capacitor, without increasing the number of terminals of the power supply control IC, is disclosed in Japan Patent No. JP-A-2007-73954 below. The invention, in order to carry out a selection from a plurality of function parameters or operation modes, attempts to realize a setting of a plurality of operation conditions with a single part by, for example, selecting the capacitance value of a specific multi-functional capacitor joined to a pin of an integrated circuit, or the like. Herein, the multi-functional capacitor, as well as setting a function parameter, operation mode, or other device characteristic in the initialization period, has some kind of normal function during a normal operation of the integrated circuit. For example, it is possible to utilize a VCC pin decoupling capacitor, feedback pin loop compensation capacitor, or the like, as a parameter/mode selection capacitor during an initialization.
In Japan Patent No. JP-A-2007-73954, in order to divert a capacitor originally disposed with another object, such as the heretofore described VCC pin decoupling capacitor or feedback pin loop compensation capacitor, to a mode setting, the capacitance value of the capacitor is determined, and the mode setting is appropriately carried out. However, after the mode setting in the initialization period is finished, the capacitor is used for the original object thereof. For this reason, the capacitance value of these capacitors has to be set to a size appropriate for realizing the original object thereof.
Herein, the original object of the feedback pin loop compensation capacitor being to compensate the frequency characteristics of a power supply control IC feedback pin loop, it is disposed so that the power supply control feedback loop in a switching power supply device does not become unstable. In the event that the capacitance value of the feedback pin loop compensation capacitor is set smaller than a value optimal for the stability of the feedback loop, the system becomes unstable and oscillates. Conversely, when the capacitance value is set larger than the optimal value, response as a power supply system is slow and, for example, it is not possible to recover the output voltage immediately even when the load fluctuates. Consequently, the capacitance value of the capacitor has a large effect on the response of the feedback system, and there are problems that restrict the design itself of the switching power supply device, such as adjustment of phase compensation being limited or the power supply oscillating abnormally, due to a change of the capacitance value.
Also, generally, the following three kinds of cases are envisaged as methods of supplying a VCC voltage to a power supply control IC. A first case makes a VCC voltage in an internal regulator from an external power supply connected to a separate terminal, as in the heretofore described Japan Patent No. JP-A-2007-73954. A second case makes a VCC voltage with a start up current generated by a start up circuit (corresponding to the internal regulator of the first case) connected to an external power source in an initialization period at a start up time, and receives a supply of the VCC voltage from an auxiliary coil at a time of a normal operation. A third case utilizes an external VCC power supply.
In order to utilize the VCC pin decoupling capacitor as a multi-functional capacitor, a regulator that generates a constant current for charging the capacitor is needed in the power supply control IC. This is because the VCC pin decoupling capacitor is connected to the VCC pin (between the VCC pin and the ground) of the power supply control IC in order to supply power to the power supply control IC itself. However, the VCC pin decoupling capacitor functions as a buffer when the power supply becomes insufficient due to the current consumption of the power supply control IC increasing, or when there is a fluctuation in the power supply voltage, and also functions so as to remove noise such as a ripple on the VCC line. Therefore, in order to realize this kind of original object, it is necessary that the capacitance value of the VCC pin decoupling capacitor is made large to a certain extent. Because of this, when attempting to charge the large capacitance value of the VCC pin decoupling capacitor with a constant current in a short period, it is necessary that the value of the constant current from the regulator is set large, and a large scale regulator has to be used in order to cause a large constant current to be generated. Consequently, a problem with cost occurs.
Also, in the third case, as power is supplied by the VCC pin being connected to an external power supply, the VCC pin decoupling capacitor is constantly in a condition in which it is connected to the external power supply. However, even when the VCC pin decoupling capacitor is charged by a constant current in this kind of connection condition, it is not possible to determine whether charging is done by a current from the external power supply, or whether charging is done by a constant current generated in the power supply control IC. That is, in order to utilize the VCC pin decoupling capacitor in a mode setting in an initialization period, it is necessary to disconnect the VCC pin from the external power supply, and a circuit for this disconnecting is provided in the exterior of the power supply control IC. Consequently, it can be said that the method of Japan Patent No. JP-A-2007-73954 is inappropriate for being applied to the third case and setting a parameter/mode.
As above, with the method of Japan Patent No. JP-A-2007-73954 utilizing a multi-functional capacitor, there are problems in that the multi-functional capacitor is difficult to use because the normal function is directly affected, it is necessary to provide a large scale regulator which leads to an increase in cost, the method may not be applicable depending on the method of supplying the VCC voltage, and the like.
Furthermore, in a separate Japan Patent No. JP-A-2007-258294, a description is given of a semiconductor integrated circuit wherein a mode switching signal is input without increasing the number of terminals. Herein, utilizing a terminal, among terminals necessary for a normal operation, wherein there is a difference between an input voltage range of a power supply voltage and that of a ground potential at a time of a normal operation, when a voltage near the power supply voltage or a voltage near the ground potential is input into the terminal, a switching of the semiconductor integrated circuit mode is carried out. Consequently, according to the technology of Japan Patent No. JP-A-2007-258294, there is provided a semiconductor integrated circuit wherein it is possible to input a mode switching signal without increasing the number of terminals.